Freddybear writes: A recent report from the US Energy Information Agency says that US carbon emissions are the lowest they have been in 20 years, and attributes the decline to the increasing use of cheap natural gas obtained from fracking wells.

Michael Mann, director of the Earth System Science Center at Penn State University, said the shift away from coal is reason for "cautious optimism" about potential ways to deal with climate change. He said it demonstrates that "ultimately people follow their wallets" on global warming.

"There's a very clear lesson here. What it shows is that if you make a cleaner energy source cheaper, you will displace dirtier sources," said Roger Pielke Jr., a climate expert at the University of Colorado.